There has hitherto been known, as a lithographic printing plate precursor, a lithographic printing plate precursor including a photosensitive image forming layer (PS plate). The PS plate basically includes two types of a negative working plate and a positive working plate. In the case of the negative working plate, a negative film is used upon exposure and then developed with a developing solution for a negative working plate. A photosensitive layer of an unexposed area is removed by this developing treatment to form a non-image area, and thus an area (exposed area) cured and insolubilized by exposure remains as an image area. In the case of the positive working plate, a positive film is used upon exposure and then developed with a developing solution for a positive working plate. A photosensitive layer of an area (exposed area) solubilized by exposure is removed to form a non-image area, and thus the unexposed area, which is not solubilized, remains as an image to form an image area.
With the progress of a computer image processing technology, there has recently been developed a method in which an image is directly written on an image forming layer by light irradiation corresponding to digital signals. An intense interest has been shown towards a CTP system in which an image is directly formed on a lithographic printing plate precursor using this method for a lithographic printing plate precursor. In a CTP plate, when the area irradiated with laser light (exposed area) is cured and becomes insoluble in an alkali developing solution to form an image area, such printing plate precursor is called a negative working plate. In contrast, when the exposed area becomes alkali-soluble and, after a developing treatment, a non-image area is formed, such printing plate precursor is called a positive working plate.
The developing solution for a positive working plate usually contains inorganic strong alkali components, for example, hydroxides, carbonates, silicates, and phosphates of alkali metals, such as sodium hydroxide, potassium hydroxide, sodium carbonate, sodium silicate, potassium silicate, sodium metasilicate, and tertiary sodium phosphate, and is a high pH aqueous solution. In contrast, the developing solution for a negative plate usually contains, as a main component, water and contains a small amount of an organic solvent, and also contains organic amines and a surfactant, although it varies depending on the composition of the image forming layer of the negative working plate. The developing solution for a negative working plate does not contain an inorganic strong alkali component and is a relatively low pH (7 to 11.5) aqueous solution.
From the viewpoint of an influence exerted on the environment, various problems have recently been pointed out regarding the use of a high pH developing solution using a strong alkali agent, thus increasing needs for a low pH developing solution using no strong alkali agent. Responding to increasing needs for the low pH developing solution, the composition of an image forming layer of a lithographic printing plate precursor has been improved so that it also dissolves in the low pH alkali developing solution. Lithographic printing plate precursors disclosed, for example, in Japanese Unexamined Patent Publication (Kokai) No. 2007-17913, Published Japanese Translation of PCT International Publication for Patent Application (Kohyo) No. 2009-524110, Japanese Unexamined Patent Publication (Kokai) No. 2009-98368, Published Japanese Translation of PCT International Publication for Patent Application (Kohyo) No. 2010-511202, and the like are lithographic printing plate precursors improved so as to be able to be treated with a low pH developing solution having a pH of 12 or lower. An image forming layer of these lithographic printing plate precursors contains no or few resin having a phenolic OH group in a structure of a phenol resin or a novolak resin. As mentioned above, the composition of the image forming layer of the lithographic printing plate precursor has been improved. However, a low pH alkali developing solution having satisfactory developability is still required regardless of whether it is a conventional developing solution for a positive working plate or developing solution for a negative plate.
The lithographic printing plate precursors comprising an image forming layer having improved composition disclosed in the above publications usually have a property, which enables easy penetration of a developing solution and also easier dissolution in the developing solution than before, so as to be able to be treated with a low pH developing solution. However, these lithographic printing plate precursors comprising an image forming layer having improved composition are likely to cause film thinning due to penetration of the developing solution into the image area when they have narrow development latitude and high development activity, and thus ink receptivity may sometimes deteriorate during printing. These lithographic printing plate precursors also have such a drawback that, if scratches exist on a surface of the lithographic printing plate before a developing treatment, the developing solution easily penetrates from the scratches and thus omission is likely to occur in the image area. These lithographic printing plate precursors also have various drawbacks that they exhibit narrower development latitude to the used developing solution after treatment of a lot of printing plate precursors (i.e. degraded developing solution), leading to decreased development speed of the exposed area and easy penetration of the developing solution into the unexposed area.